1. Field of the Invention (Technical Field)
The present invention relates to linear optical devices used to effectuate switching.
2. Background Art
Since lasers have become routinely utilized, there has been considerable interest in the development of optical computing devices. In modem communication links, the requirement for both a high data rate and small error probability requires implementation of all-optical devices. It has become widely accepted that optical networks should fully replace the traditional electronics.
All-optical devices have the advantage of such important properties of light as high frequency, broad spectral range, high speed and capability of parallel processing. Various applications of optical computing and its essential components have been presented, including pattern recognition, polarization-encoding, optical interconnects, and logic gates which were initially based on spatial filtering. Among these, optical gates and/or optical switches are key devices for all-optical networks. For example, ultrafast switches are very attractive for time division, multiplexers and demultiplexers apart from being the basic logic elements of future computing devices.
The operating principles of all-optical elements can differ significantly from one realization to another. (the different techniques are compared by their performance in Table 1.) However, several common features unify them. A signal beam of a definite polarization or a known frequency is used. Upon propagation through some optically active medium or a waveguide with variable characteristics, the initial polarization and/or the frequency of the signal beam changes. Generally, the logic element is built on the state of the polarization or the frequency change for the signal beam, while the initial signal state serves as a reference point. Some methods exploit optical anisotropy of organic molecules, while others utilize the nonlinear properties of a medium. An alternative approach is the implementation of polarization rotation.
The variations of medium characteristics can be caused by absorption/gain saturation and/or a non-linear phase shift, or by some other mechanism. These variations are typically associated with a refractive index change induced by the applied electromagnetic field(s) and/or their interactions with the medium. To control the logic element performance, a command link, such as an external source linked to variation of medium characteristics to change of the signal state, is usually formed.
A modified Mach-Zehnder all-optical switch (see Table 1) utilizes a semiconductor band-filling effect. A phase difference between two components of the signal beam is obtained in a non-linear waveguide, and the subsequent polarization discrimination is implemented. Although rise and fall times of 1.3 ps can be obtained (by ultrafast pulse excitation) and are not limited by the slow relaxation time of induced optical non-linearities, there exist several impeding factors. In spite of reaching switching speed (on-off time) of 40 ps, the repetition rate is only approximately 12 ns, which is approximately three orders of magnitude larger. The non-linear refractive index change, induced in the waveguide, is not constant with time. Therefore, it is not clear how stable this switch is over time intervals exceeding several cycles. The low light coupling efficiency, typical for many non-linear processes, is estimated to be approximately 10% and brings up the question of the stability of the optical non-linearities.
The amplifying optical Kerr gate overcomes the problem of low efficiency as the maximum transition intensity gain reaches 40 at a wavelength of 650 nm. This is due to amplification in a non-linear Kerr medium composed of organic molecules and laser dyes with intrinsic anisotropy. Nevertheless, the pulse width of such a gate is only about 20 ps and is limited by the reorientation time of the dye molecules. Reorientation time represents the repetition rate, during which the signal amplification can be obtained, and during which the Kerr gate operates successfully. In order to increase the time interval, the polarization must be adjusted as well as the amplitude of a closing pulse, but such a procedure is very delicate and critical. The alternative method of finding non-linear materials with much longer relaxation times is still the subject of future research.
Other methods, shown in Table 1, exploit a signal beam scattering from dynamic gratings induced in a non-linear medium due to the presence of second or third-order susceptibility. An AND gate based on four-wave mixing in a semiconductor laser amplifier can provide two 10 GBit/s data streams with an error bit rate less than 10.sup.-10. However, gate operation is limited by the nanosecond time interval, during which the induced grating exists. Another all-optical AND gate results in a signal-to-noise ratio of 17 dB. Due to high waveguide losses and the lack of diffraction in the planar direction, the second-harmonic generated signal is only about 150 nW, which shows the low efficiency of the process. Another disadvantage of both gates is their slow repetition rates.
A more promising alternative is the implementation of polarization rotation. Polarization saturation spectroscopy examines the interaction of counter-propagating beams in a moderately dense atomic/molecular vapor. Due to the atomic/molecular interactions with laser beams and subsequent changes in the polarization, a significant amount of optical birefringence is introduced so that there can be large differences in transmission. Beam transmission controlled by another laser can be applied for some form of an all-optical device.
Different aspects of polarization rotation phenomena have been studied, especially the effects in alkali vapors. A resonant birefringence due to the optically induced level shifts and optical dichroism under the direct resonance absorption have been suggested. The polarization rotation induced by resonant two-photon dispersion in sodium vapor has also been studied. Self-induced optical activity under resonant and non-resonant conditions in rubidium vapors have been observed experimentally. An optical NOR gate using another alkali vapor, cesium, has also been demonstrated.
Although published less than one year before the priority date of this application, the New Mexico State University thesis entitled Optical Switching in Atomic Vapors: Theoretical Model of an All-Optical AND Gate, by Evgeni Yurij Poliakov, provides useful background information and is herein incorporated by reference.
The following patents disclose all optical devices and related subject matter but are quite different from the present invention: U.S. Pat. No. 5,771,117, to Harris et al., entitled Method and Apparatus for Nonlinear Frequency Generation Using a Strongly-Driven Local Oscillator; U.S. Pat. No. 5,710,845, to Tajima, entitled AH-Optical Switch; U.S. Pat. No. 5,272,436, to Chaillout et al., entitled Optical Pumping, Resonance Magnetometer Using a Light Beam with Controlled Polarization; U.S. Pat. No. 5,268,785, to Crenshaw et al., entitled All-Optical Switch Utilizing Inversion of Two-Level Systems; U.S. Pat. No. 5,076,672, to Tsuda et al., entitled All-Optical Switch Apparatus Using a Nonlinear Etalon; U.S. Pat. No. 4,918,699, to Boyd et al., entitled System for Combining Laser Beam by Transfer of Energy Therebetween in Atomic Vapor, U.S. Pat. No. 4,656,439, to Wessel, entitled System for Nanosecond Modulation of an Infrared Laser Beam by Coherent Stark Switching; U.S. Pat. No. 4,406,003, to Eberly et al., entitled Optical Transmission System; U.S. Pat. No. 3,720,882, to Tang et al., entitled Parametric Frequency Conversion; and U.S. Pat. No. 3,667,066, to Kastler et al., entitled Optically Pumped Alkali Atomic Beam Frequency Standard.
Optical devices based on nonlinear optical processes are typically characterized by low efficiency. Another disadvantage of such devices is low repetition rates. The present invention is an all optical multiplexer based on linear polarization rotation in alkali vapors and does not induce optical nonlinearities.
This approach is based on linear interactions of the laser radiation with an active medium of alkali atoms. Fast and efficient optical switching can be reached in a linear regime. Fast rise times and fast fall times are both necessary. To increase speed, stimulated induced emission of the excited atoms to the ground state effectively reduces the relaxation time of the atoms. Fast oscillations, obtained through the interactions of a pump beam with the alkali atoms controls the polarization rotation and the transmission of a linearly polarized signal beam. Specifically, a circularly polarized pump is tuned to exact resonance of the S.sub.1/2.fwdarw.P.sub.1/2 (J.sub.1/2.fwdarw.J.sub.1/2) transition of alkali vapors. Multiplexers and other forms of digital logic are accomplished with this methodology. Excited atoms are driven with rates that are much faster than the spontaneous relaxation rate.
TABLE 1 Performances of various all-optical elements. Different Types of Gates/Switches "Fast" Ultrafast AND Gate on Programmable Proposed Performance Kerr Mach-Zehnder Four-Wave AND Gate Characteristics Gate Switch Mixing Gate Model Data Rate per Channel 50 Gbit/s 0.1 Gbit/s N/A 10 Gbit/s 30-50 Gbit/s Signal Gain 2.4-40 &lt;0.1 10.sup.-3 10.sup.-5 0.25-4.0 Response Times: a) rise time 2.5 ps 1.3 ps est. 50 ps N/A .about.10-15 ps b) fall time 5-15 ps 1.3 ps est. 50 ps N/A .about.10-15 ps Repetition Rate 20 ps 12 ns est. 100 ps 1 ms* .about.20-35 ns Saturation Time N/A N/A nanoseconds** nanoseconds** .about.200 ns Pump Requirements: a) laser operation pulse pulse continuous pulse continuous b) pulse duration .about.1 ps 40 ps N/A 100 ps N/A c) power .about.20 MW .about.125 mW 10 mW 0.24-0.3 W est. 1.0 W Extinction Ratio N/A N/A &gt;100 N/A 200 Operating Wavelength 650 nm 880 nm 1548.8 nm 532 nm 400-780 nm *- due to Q-switching technique **- time is limited by the existence of the dynamic gratings.